Automatic frequency control circuits



June 21, 1938. D. E. FOSTER AUTOMATIC FREQUENCY CONTROL CIRCUITS 2Sheets-Shet l Filed A ril 3,1956

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TOJIE D. E. FOSTER AUTOMATIC FREQUENCY CONTROL CIRCUITS 2 Sheets-Sheet 2Filed April AMPL.

Allllll Alllll I KC OFF RESONANCE ATTORNEY Patented June 21, 1938 UNITEDSTATES PATENT OFFICE AUTOMATIC FREQUENCY CONTROL CIRCUITS wareApplication April 3, 1936, Serial No. 72,495

8 Claims.

My present invention relates to automatic frequency control circuits forsuperheterodyne receivers, and more particularly to improved andefficient discriminator and demodulation net- 5 works for automaticfrequency control circuits used in superheterodyne systems.

In my application Serial No. 55,749, filed December 23, 1935, there hasbeen disclosed an automatic frequency control system of a practical 10and eiiicient type. Generally, such a system comprises a double dioderectifier discriminator network adapted to produce from the IF energy,direct currentvoltage for AVC and AFC purposes, as well as audio voltagefor the audio network 15 of the receiver. l-leretofore the use of AFChas been recommended only on superheterodyne receivers having more thanone stage of IF amplification because of selectivity limitations of thediscriminator system using a two-winding transformer.

It may be stated that it is one of the important objects of my presentinvention 'to provide an AFC system for a superheterodyne receiverwherein the frequency discriminator network 25 employs three windingtransformers with which the signal selectivity is not reduced, butinstead is increased; it being pointed out that it is one of theessential characteristics of the present improvement to utilize anindependent diode 30 rectifier for the production of the AVG and audiovoltages.

Another important object of this invention is to provide an AFC systemin a superheterodyne receiver equipped with a single stage of IF am- 35plification, satisfactory selectivity being secured in such a receiverby the use of a third winding on the discriminator transformer, whichwinding feeds adiode rectifier functioning to produce audio voltage.

40 Another object of the invention may be stated to reside in theprovision of a superheterodyne receiver of the type employing AFC, andthe discriminator of the control system comprising a pair of coupledresonant circuits each tuned to 45 the operating IF, and a thirdresonant circuit,

tuned tothe operating IF, being coupled to the second of thediscriminator circuits and feeding a demodulator diode.

Still other objects of my present invention are 50 to improve generallythe efiiciency of AFC systems for superheterodyne radio receivers, andmore especially to provide frequency control systems in a manner suchthat they may be economically and readily embodied in commer- 55 cialbroadcast receivers.

The novel features which are believed to be characteristic of myinvention are set forth in particularity in the appended claims; theinvention itself, as to both its organization and method of operation,will best be understood by reference to the following description, takenin connection with the accompanying drawings in which I have indicatedvarious circuits whereby my invention may be carried into effect.

In the drawings:

Fig. 1 schematically shows a circuit diagram embodying a preferred formof the invention,

Fig. 2 is a circuit diagram of a portion of the receiving circuit ofFig. l, and showing another embodiment of the invention, 1

Fig. 3 illustrates a further embodiment, and

Fig. 4 graphically shows the operation of the discriminator networkillustrated in Fig. 1.

Referring now to the accompanying drawings, wherein like referencecharacters in the different figures designate similar circuit elements,attention is first directed to Fig. 1, which'shows in a purely schematicmanner a superheterodyne receiver embodying AFC. Since the functioningof the present invention is not dependent in any way upon the particularconstruction of the superheterodyne receiver, or the specific frequencycontrol tube circuit employed therewith, it is believed sufficient forthe purposes of this application to point out a typical superheterodynereceiving system which can be utilized in conjunction with the noveldiscriminator and audio demodulator construction of my presentinvention. The receiving system shown in Fig.

1 is a conventional representation of a system shown in Fig. l of myaforesaid copending application. Attention is further directed to Fig.

4 of application Serial No. 45,413, filed Oct. 17, 1935 by S. W. Seeley,for a detailed circuit diagram of a receiving system whose generalconstruction is similar to that shown in Fig. 1.

It will be sufficient for those skilled in the art briefly to describethe main elements of a superheterodyne receiving system which isprovided with AFC. The usual signal carrier energy collector A iscoupled to the tunable input circuit of the radio frequency amplifier ofthe receiver. The tunable input circuit usually comprises a variabletuning condenser l, and it is to be clearly understood that the numeral2 may designate one or more stages of RF amplification, each of whichstages may be tunable by a variable condenser. In order to preservesimplicity of disclosure and drawings, the numeral 3 designates thefirst detector, and its variable tuning condenser is designated by thenumeral 4.

The output circuit 3 of the first detector is resonated to the operatingIF, and the latter may have a value chosen from a range of 175 to 465kc. The IF amplifier 4' has its input circuit 5 resonated to theoperating IF, and is coupled to the first detector output circuit 3. TheIF amplifier 4' is followed by a double diode tube 5, and this tube maybe of the 61-16 type. This type of tube is provided with independentdiode electrodes, and the common resonant input circuit I has one sidethereof connected to the diode anode 8, while the opposite side of thecircuit is connected to the diode anode 9. The high alternatingpotential side of the output circuit 6, of the IF amplifier 4', isconnected through condenser I ll to the midpoint of the secondary coil Iof input circuit I. The midpoint II is connected to the junction ofresistor portions I2 and I3; one side of resistor I2 being connected tothe cathode 8' of the diode 8-8, and one side of resistor I3 beingconnected to the cathode 9' of the diode 99.

The condenser I4 is connected between cathodes 8 and 9', and the cathode9' is grounded. The input circuit I is tuned to the operating IF, and isreactively coupled to the circuit 6 as designated by the referenceletter M. The AFC network involves the tunable tank circuit I8 of thelocal oscillator I9. As is well known to those skilled in the art, thevariable tuning condenser 20, in the tank circuit I 8, has its rotorsmechanically uni-controlled with the rotors of the variable tuningcondensers of the tunable signal circuits feeding the first detector.The dotted line 2i represents such mechanical uni-control. Of course,the oscillator I 9 is tuned at any setting of the tuning mechanism 2! toa frequency which differs from the frequency of the signal circuits bythe operating IF. Those skilled in the art are, also, fully aware of themanner of employing padding condensers in the tank circuit 18 formaintaining the operating IF constant in value as the tuner 2| is variedthrough the operating frequency range, and the latter may be thebroadcast range of 500 to 1500 kc. It may even be in the short-wavebands where the receiver is of the multi-range type.

The locally produced oscillations are impressed on the first detector 3in any desired manner, as by impressing them on the cathode circuit ofthe first detector. Of course, it is not essential to this invention toemploy separate tubes 3 and I9 for the mixer and oscillator functions.For example, a pentagri-d tube of the 2A? type may be employed, in anywell known manner, in a composite oscillator-mixer stage. In any case,there is electrically associated with the tank circuit I8 a frequencycontrol tube 22. The electrical connections between the plate circuit oftube 22 and tank circuit I8 are such that an inductive reactance isreflected across the tank circuit.

Since it is not important for the purposes of the present invention toknow of the construction of the frequency control tube, it is notbelieved necessary to go into any detailed discussion of the specificelectrical connections between the frequency control tube 22 and thetank circuit I8. In my aforementioned application such a detailedexplanation is given; and the same is equally true of the aforesaidSeeley application. In general, it may be said that the tube 22 may beconnected to the tank circuit I8 so as to produce an effectiveinductance across the tank circuit I8. The magnitude of this effectiveinductance is, of course, a function of the mutual conductance of tube22. The AFC connection 26 is made from the control grid of tube 22,through an appropriate alternating current filter element not shown, tothe cathode side of resistor I2. The mutual conductance of tube 22 isvaried in dependence upon the magnitude of the direct current componentof the differential rectified IF energy. The magnitude and polarity ofthe potential at the cathode side of resistor l2 determines the sense ofmagnitude variation of the effective inductance reflected across tankcircuit I8 by control tube 22. The reference letter E2 denotes that AFCvoltage applied through lead 25 to the frequency control tube 22.

The theoretical basis for the production of the AFC voltage E2 residesin the following considerations. The potentials at either end of coil I,with respect to the center tap I I, are 180 degrees out of phase. Hence,if the center tap I I is connected to the primary circuit 6, a potentialis realized which maximizes above the resonant frequency of circuit 6and I, and a second potential is realized which maximizes below thiscommon resonant frequency. If these two potentials are applied to apairof rectifiers, such as the diodes in Fig. 1, and the resulting directcurrent voltages are added in opposition, the sum will be equal to zero.The output load of the two diodes comprises the resistors I2 and I3which are of like magnitude, and the latter are connected in seriesbetween the cathodes 8' and 9'.

In the type of discriminator network shown in Fig. 1 the primary andsecondary circuits 6 and I are so connected that two vector sumpotentials of the primary and secondary voltages may be realized. Pointa of circuit 6 is at the same alternating potential as point I I due tothe connection through the large condenser It]. At IF resonance thephase of point a with respect to ground potential is zero; point II 'is,therefore, at zero phase. The current distribution about point II isequal; at any given instant point 0 is as much positive as point d isnegative. The voltages impressed on the diode rectifiers are thereforeequal, but opposite in phase. Since the rectifiers are in seriesopposition the potential E2 is zero at resonance. If now the signalenergy departs from resonance, a phase shift of 90 degrees(approximately) occurs in the circuit. The voltages induced in the twohalves of coil I are still equal in magnitude and opposite in phase withrespect to point I I. The voltage drop across circuit 6 is now addedvectorially to the induced voltages. Thus, the potential at one side ofthe secondary, say point 0, will be the sum of the induced voltage (II0) and voltage across 6; While the potential of the other side, point(1, will be the difference between the drop in circuit 5 and the voltageinduced in secondary position IId.

, In the last case, then, the input voltage to the upper rectifier ismuch greater than that to the lower one. Hence, the voltage drop acrossresistor I2 will be greater than that across I3, and the cathode end ofresistor I2 will be positive with respect to the grounded end ofresistor I3. When the signal frequency impressed on primary circuit 6 isoff resonance in the opposite direction, the cathode end of resistor I2becomes negative with respect to ground. The sense of detuning thusdetermines the polarity .of the cathode end of resistor I2; the amountof detuning determines the magnitude of the AFC bias. As stated beforethe magnitude and the polarity of the potential at the cathode side ofthe resistor 12 determines the magnitude of the reactance reflectedacross tank circuit l8 by tube 22. If the AFC voltage applied to thegrid of tube 22 is positive (thereby overcoming some of the initial biasapplied in the cathode circuit of that tube) its mutual conductance isincreased. This, in turn, acts as though the tuning condenser 20 hadbeen decreased in value thereby causingthe tuned frequency of the tankcircuit i8 to increase. It will now be seen that the frequencydifference be tween the signal and oscillator circuits is madeautomatically to shift towards the desired IF value as the receiver istuned towards a desired station setting.

The AFC action commences as soon as a little of the energy of a carrierwave is applied to the primary circuit 6. The polarity of the AFCvoltage E2 with respect to ground depends on the phase of the couplingM. By way of example, it is pointed out that in Fig. 1 the coupling Mmay be phased so that E2 becomes positive with respect to ground whenthe applied signal is lower than the desired center frequency ofcircuits'li and l. Heretofore the use of AFC has been rec ommended onlyfor superheterodyne receivers having more than one stage of IFamplification because of the selectivity limitations of a discriminatorsystem using a two-winding transformer. For example, as shown in myaforesaid copending application, the AFC, AVC and audio voltages areobtained from the resistors 52-43 of the discriminator network, but theaudio selectivity of the discriminator is low so that two stages of IFamplification are required to secure adequate selectivity.

However, when a superheterodyne receiver is used, such as shown in Fig.1, where it is desired to use but a single stage of IF amplification, itis necessary to resort to the present invention to accomplishsatisfactory results. I have found that satisfactory selectivity can besecured in a superheterodyne receiver having only one IF amplifier, bythe use of a third winding, on the discriminator transformer, feedinganaudio diode. In working out a practical embodiment of this invention,however, it must be kept in mind that selectivity is only one of thecharacteristics of a discriminator circuit which are of interest inconsideration of AFC action. In addition to audio selectivity, thefollowing characteristics are also important: audio gain; AFC peak gain;AFC slope; and frequency separation of the AFC peaks. The audio, orsignal, gain may be expressed as the ratio of peak value of signal inputon the grid of the IF amplifier driving the discriminator to the directcurrent voltage in the audio diode.

The AFC voltage reaches a maximum in the positive sense at somefrequency off the center frequency, and a maximum in the negative senseon the opposite side of the center frequency. The magnitude andseparation of these two AFC peaks, and the slope of the characteristicpassing through the center frequency, are of importance in discriminatoraction. If the peaks are too widely separated, the slope will be small,and if they are too close together signal modulation may be heard,before the AFC acts, as the frequency is varied. In a discriminatornetwork using a circuit of the type shown in my aforesaid copendingapplication, and wherein the audio voltage is taken off from themidpoint of resistors 12 and. I3, it can be shown, operating with an IFof 460 kc., that the voltage selectivity, across resistor 13, is low.

In place of using the voltage E3 developed across resistor l3 as theaudio and AVC voltages, a separate diode rectifier 3B is employed. Thisrectifier is provided with a tuned input circuit 3|, and the tertiarytuned circuit 3| is coupled magnetically, as at M2, to the coil 1. Thecoupling is arranged so that coil 32 of the tertiary circuit 3! iscoupled only to coil 1', and not to the primary circuit 6. This is done,as schematically shown in Fig. 1, by using a few coupling turns 32 closeto coil 1'. These turns are shown at the center of coil 1' and thisarrangement should be followed physically in order to keep the capacitycoupling of coil 32' symmetrical with respect to both sides of coil 1'.In order to keep the capacity coupling small and symmetrical, it isdesirable to keep coil 32 and the leads thereto well separated from coil1; possibly even to the extent of putting coil 32' in a separate shield.The load resistor M1 is connected in series between the ground side ofthe cathode of diode 3i! and the coil 32, a by-pass condenser beingconnected in shunt across the resistor 40.

The AVC connection including proper alternating current filterresistors, is made to the grid circuits of the signal transmission tubeswhose gain is to be automatically regulated. As shown in Fig. 1, the AVGconnection 4! is made between the grid circuits of amplifier 2, firstdetector 3 and the anode side of the resistor Ml.

The grid circuit of IF amplifier 4 is connected by AVC lead 4| to anintermediate point on load resistor ii]. The audio voltage developedacross resistor M) is transmitted to an audio frequency utilizationnetwork which may employ one, or more, stages of amplification followedby a reproducer. The voltage developed across the resistor 60 is denotedby the reference letter E1.

The relations between El; E2 and E3 are graphically shown in Fig. 4. Inthe latter figure, there is plotted kc off resonance against volts. Asexplained before, the discriminator network has little selectivity whenthe audio signalis taken from the center tap of the diode resistors l2and I3, since the discriminator is primarily a phase responsive network.However, the discriminator acts as any tuned circuit, of the same Q andcoupling, to a third circuit coupled to it, and the selectivity of thisthird circuit is, therefore. excellent. Capacity coupling dissymmetryproduces departure from symmetry of both signal and AFC selectivity. Forthis reason, the physical arrangement between coil 32 and coil 1recommended above is of advantage.

The following illustrative circuit constants are given, but it is to beclearly understood that they are purely illustrative in nature, and aresupplied solely to enable those skilled in the art to readily practicethis invention: I

lVI30 mh. M2-17.2 mh. Condenser +100 mmf.

Resistor 400.25 megohm Condenser 140.1 mf.

Resistor 12--0.5 megohm Resistor 13-0.5 megohm The tertiary circuit 3iacts to decrease, however, the AFC slope and separates the AFC peaks,

since it acts as a resonant absorption circuit. Re-' duction of tertiarycoupling decreases this effect, but also acts to decrease the signalcircuit gain. While the slope of the AFC characteristic is somewhatdecreased, the AFC gain is good. This arrangement for the discriminatornetwork, and audio demodulator, gives satisfactory characteristics for asuperheterodyne receiver using a single stage of IF amplification.

The use of separate diodes for AFC and audio demodulation imposesadditional power output requirements on the IF amplifier driving thediodes. Voltage for the AVG action may be derived either from the centertap of the discrimination resistors I2 and I3, or from the load resistor49. As shown in Fig. 4, the voltage E3 is not symmetrical with respectto resonance, so that if this voltage is used for AVC purposes, the gainof the receiver will be higher on one side of the IF resonant frequencythan on the other; producing slight asymmetry.

By using voltage E1, derived from the signal circuit for AVC purposes itwill be symmetrical about resonance. This is clearly shown in Fig. 4,and the voltage E1 derived from across resistor 0 will not be subject tothe asymmetry noted above. At normal bias the IF stage 4 will drive thediode satisfactorily but at high bias such as would be produced by fullAVC control on the IF stage, curvature of the characteristic will occur.Approximately 20 volts will be required for AVC action in order tosecure adequate control of large signals, assuming 3-tubes are to becontrolled. In order that the audio output of the signal diode 30 shallbe free from distortion, the characteristic should be straight. Therange of signal inputs over which the characteristic is straight,determines the modulation percentage which may be handled Withoutdistortion. With 20 volts 1F bias, 5.36 volts carrier will be requiredto derive 20 volts of AVG bias. For 100% modulation twice this input isinstantaneously applied, and for such applied signal the characteristicwill depart appreciably from a straight line. However, if only half ofthe developed AVC voltage is used on the IF tube 4', or 10 volts, whenthe total developed AVC bias is 20 volts, then 1.6 volts of signal isrequired. Under these conditions, the characteristic will be very nearlya straight line up to the 3.2 volts input required for 100% modulation.Accordingly, by applying one half AVC' bias to the IF amplifier 4 whichdrives the diodes, and full AVC bias to the preceding tubes largesignals can be handled without appreciable distortion, and using thediscriminator network shown in Fig. 1.

In Fig. 2, there is shown a variation of the discriminator-demodulatornetwork. Here the tertiary tuned circuit 3! of diode demodulator 36 iscoupled, as at M1, to the primary circuit 6. The secondary circuit I hasan effect on the signal circuit selectivity nevertheless. The efiect ofthe secondary on the tertiary is to act as an absorption circuit atresonance; thereby giving a double peak characteristic to voltage E1. IfM is reduced to minimize this effect, the AFC gain will be de creasedand the AFC peaks approach relatively close to each other, even lessthan 8 kc. separation. Reduction of M1 to improve selectivity decreasesthe gain of the signal circuit. While the AFC gain (about 50) and slope(approximately 28 volts per kc. per peak volt input) are excellent, thesignal circuit gain and selectivity are not nearly as good as those fora simple diode circuit. This connection is desirable for high fidelityreceivers, since selectivity is relatively good but high audio frequencyside bands are not unduly attenuated.

In Fig. 3, there is shown a modification of thediscriminator-demodulator network of Fig. 1,

wherein the tuning of the tertiary circuit 3| is omitted. The coil 32'vof circuit 3| is magnetically coupled to secondary circuit 1. It isimportant, as in the previous circuits, to keep the capacity couplinglow and symmetrical with respect to the discriminator. The choke 50 isconnected between the mid-tap of resistors 52 and I3 and the mid-pointof coil 1'. In this network, mere- ].y by way of example, coil 53 has avalue of 15 mh.; M may be 24 mh.; M3 is 490 mh.; shunt condenser 5| mayhave a value of 100 mmf.

This network has less AFC gain than that of Fig. 1 (32 as compared to59), but is steeper in AFC slope (12.8 compared to 8.8). The network ofFig. 1 has a signal gain at resonance of 34 and AFC peak separation of13 kc.; the network of Fig. 3 has corresponding values of 38.5 and 11kc. Further, the signal selectivity for the network of Fig. 3 is lessthan that of Fig. 1. However where such less selectivity can betolerated, the network of Fig. 3 is satisfactory.

The diode 3!! may be the diode section of a diode-triode tube, such asone of the '75 or 6Q? type. Since a 6H6 type tube will normally be usedfor tube 5', the use of the diode section of the first audio amplifiertube involves the use of no additional tube over the type of circuitshown by me in Fig. 1 of my aforesaid co-pending application. In otherwords, it will be understood that the numeral 33 designates a tube ofthe well known multiple function type wherein the diode sectionfunctions as the diode demodulator and AVC rectifier, and the amplifiersection of the tube is fed with the audio component of the voltagedeveloped across resistor 40.

It is also to be clearly understood that the present invention is in noway dependent upon the particular nature of the frequency controlnetwork electrically associated with the oscillator tank circuit. Thespecific type of frequency control tube which has been disclosed in thisapplication is merely shown by way of illustration. From the aforegoingdisclosure it will be seen that with the characteristics of thedescribed AFC systems using triple winding transformers, a,superheterodyne receiver with a single IF amplifier stage may use such acontrol system and still maintain adequate selectivity.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaims.

What I claim is:

1. In a superheterodyne receiver of the type provided with a singleamplifier having input and output circuits resonated to an operatingintermediate frequency, a first detector network whose output iselectrically associated with the amplifier input, a local oscillatornetwork provided with a tank circuit and electrically associated withthe first detector network, an automatic frequency control circuitelectrically associated with the amplifier output circuit and theoscillator tank circuit, said control circuit comprising a pair ofresonant circuits reactively coupled in cascade and tuned to theoperating intermediate frequency, a pair of rectifiers operativelyassociated with the second of said resonant circuits in seriesopposition to produce a direct current voltage whose magnitude andpolarity is d p nden u on t e-f q n y d p ure of the 9 29 1 n r y ofsaidfi s detectorv netw k from ai p r ting med eq ency, hetfi st of saidcascaded'circuits being said amplifier output circuit, 'a loadimpedance, said rectifiersbeing in series opposition acrosstheimpedance, means establishing one terminal of the impedance at afixed potential, means establishing the midpoint of the second of saidcascaded circuits at the alternating potential of the high potentialside of the'amplifier output circuit, means connectingthe impedancemidpoint to said first midpoint, and means deriving said direct currentvoltage from the opposite terminal of said impedance, an additionalrectifier having a signal input circuit reactively coupled to at leastone of said; cascaded circuits, an audio utilization networkelectrically coupled to said additional rectifier, and an automatic gaincontrol circuit electrically associated with; at least one of the signaltransmission; tubes preceding said cascaded circuits,;said: gain controlcircuit being connected to said additional rectifier for derivingtherefrom a direct current component of rectified signal energy. v

2. In combination with a source of electrical high frequency waves of apredetermined frequency, an automatic frequency control circuitcomprising a pair of cascaded reactively coupled resonant circuits, saidcircuits being tuned to said predetermined frequency, a pair ofrectifiers connected to the second of the cascaded circuits in such amanner that said second circuit acts as a common input circuit for therectifiers, a

', common load impedance for the rectifiers, said rectifiers'being inseries opposition across the impedance whereby their direct currentoutputs are in opposition, means for utilizing the resultant directcurrent of said opposed rectifiers, and additional rectifier having aresonant input circuit, tuned to said predetermined frequency,reactively coupled to the second of said cascaded circuits, means forutilizing the audio component of signal currents rectified in saidadditional rectifier circuit.

3. In combination with a source of electrical high frequency waves of apredetermined frequency, an automatic frequency control circuitcomprising a pair of cascaded reactively coupled resonant circuits, saidcircuits being tuned to said predetermined frequency, a pair ofrectifiers connected to the second of the cascaded circuits in such amanner that said second circuit acts as a common input circuit forthe'rectifiers, a common load impedance for the rectifiers, saidrectifiers being in series opposition across the impedance whereby theirdirect current outputs are in opposition, means for utilizing theresultant direct current of said opposed rectifiers, an additionalrectifier having a resonant input circuit, tuned to said predeterminedfrequency, reactively coupled to the second of saidcascaded circuits,means for utilizing the audio component of signal currents rectified insaid additional rectifier circuit, each of said rectifiers being adiode, and a link coil coupling the input circuit of said additionalrectifier to'the said second of the cascaded circuits.

4. In a radio receiver, a coupling transformer comprising a pair ofresonant circuits reactively coupled in cascade, said circuits beingtuned to a predetermined operating frequency, a pair of rectifiersconnected to the second of said cascaded circuits so that the secondcircuit acts as a common input circuit for the rectifiers, meansestablishing the midpoint of the second circuit at thealternating'potential of the high potential sideof the first of thecascaded circuits, a common load impedance for the rectifiers, saidrectifiers being in series opposition across the impedance, meansestablishing one terminal of the impedance at a relatively fixedpotential, means establishing the impedance midpoint at the potential ofsaid first midpoint, and means for deriving from the opposite terminalof the impedance the resultant direct current which varies magnitude andpolarity, dependent upon the departure of an applied radio frequencyfrom the predetermined operating frequency, a third signal energy inputcircuit reactively coupled to one of said cascaded circuits, and a thirdrectifier coupled to the third circuit so that the resultant current ofsaid third rectifier varies in magnitude, but does not vary in polarity,as the frequency applied to said coupling transformer is varied from thepredetermined operating frequency. i

5. In a superheterodyne receiver of the type' providedwith asingleamplifier having input and output circuits each resonated to anoperating intermediate frequency, a first detector network having anoutput circuit coupled to said input circuit, a local oscillator networkhaving a tank circuit, an automatic frequency control network connectedbetween said amplifier output circuit and said oscillator network, saidcontrol network comprising a pair ofrectifiers having a common inputcircuit resonated to said intermediate frequency, said cornmoninputcircuit being reactively coupled to said amplifier output circuit incascade, means for establishing the midpoint of said latter circuit atthe alternating potential of the high potential side of said amplifieroutput circuit, a common load impedance for said rectifiers, the latterbeing in series opposition across said impedance, means for establishingthe midpoint of said impedance at the same potential as said firstmidpoint, means establishing one end of the impedance at a relativelyfixed potential, means connected to the opposite end of the impedancefor deriving a direct current voltage whose magnitude and polarity isdependent upon the frequency departure of the first detector'outputcircuit energy from the operating intermediate frequency, an auxiliaryrectifier having an input circuit coupled to at least one said cascadedcircuits to receive intermediate frequency energy therefrom, and autilization network coupled to the auxiliary rectifier to utilize therectified current of the latter.

6. In a superheterodyne receiver of the type provided with a singleamplifier having input and output circuits each resonated to anoperating intermediate frequency, a first detector network having anoutput circuit coupled to said input circuit, a local oscillator networkhaving a tank circuit, an automatic frequency control network connectedbetween said amplifier output circuit and said oscillator network, saidcontrol network so I as said first midpoint, means establishing one endof the impedance at a relatively fixed potential, means connected to theopposite end of the impedance for deriving a direct current voltagewhose magnitude and polarity is dependent upon the frequency departureof the first detector output circuit energy from the operatingintermediate frequency, an auxiliary rectifier having an input circuitcoupled to at least one said cascaded circuits to receive intermediatefrequency energy therefrom, a utilization network coupled to theauxiliary rectifier to utilize the rectified current of the latter, saidutilization network being adapted to utilize the audio component of therectified current, and said auxiliary rectifier input circuit beingcoupled to the said common input circuit of said pair of rectifiers.

7. In a superheterodyne receiver of the type provided with a singleamplifier having input and output circuits each resonated to anoperating intermediate frequency, a first detector network having anoutput circuit coupled to said input circuit, a local oscillator networkhaving a tank circuit, an automatic frequency control network connectedbetween said amplifier output circuit and said oscillator network, saidcontrol network comprising a pair of rectifiers having a common inputcircuit resonated to said intermediate frequency, said common inputcircuit being reactively coupled to said amplifier output circuit incascade, means for establishing the midpoint of said latter circuit atthe alternating potential of the high potential side of said amplifieroutput circuit, a common load impedance for said rectifiers, the latterbeing in series opposition across said impedance, means for establishingthe midpoint of said impedance at the same potential as said firstmidpoint, means establishing one end of the impedance at a relativelyfixed potential, means connected to the opposite end of the impedancefor deriving a direct current Voltage whose magnitude and polarity isdependent upon the frequency departure of the first detector outputcircuit energy from the operating intermediate frequency, an auxiliaryrectifier having an input circuit coupled to at least one said cascadedcircuits to receive intermediate frequency energy therefrom, and autilization network coupled to the auxiliary rectifier to utilize therectified current of the latter, said auxiliary rectifier input circuitbeing resonated to said intermediate frequency and being coupled to thesaid common input circuit, and said utilization network including'meansfor impressing at least a portion of the direct current component ofsaid rectified current upon said amplifier as a gain control potential.

8. In combination with the local oscillator and intermediate frequencyamplifier networks of a superheterodyne receiver, an automatic frequencycontrol circuit, responsive to frequency shifts in the amplifier signaloutput energy from an assigned intermediate frequency, for impressing onthe oscillator a frequency correction voltage dependent upon saidshifts, said control circuit being of the type including a pair ofresonant circuits, tuned to the assigned frequency, in cascadesubsequent to the amplifier, an audio demodulator network comprising arectifier having an input circuit coupled to the second of the cascadedpair of circuits, said last input circuit being tuned to said assignedfrequency whereby the selectivity at that circuit is increased, andmeans for utilizing the audio componentof the rectifier output current.

'DUDLEY E. FOSTER.

